Hi, may be the images that you are using hold the space, incus image list shows the images on your system.
Ohh pardon me, you have just one image. 
What is the lxd version? I’m not sure but it may be btrfs problem. Humm same symptom i suppose.
https://discuss.linuxcontainers.org/t/no-space-left-on-device-while-changing-the-btrfs-storage-pool-of-an-lxd-container-with-docker/10040/4
Regards.